Faulty GU10 holder needs replacing.
Shower is not electric.
Regulations (wrt RCDs) applying to changed and new installations do not retrospectively apply to existing installations.
Conclusion:
1. OP can buy another GU10 holder and fit it, as long as it's IP65 rated.
2. OP shouldn't replace MCB with RCBO without more experience, knowledge and the ability to test the circuit.
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